Modern communications systems often employ linear modulation schemes that exhibit high peak to average power ratios to achieve high capacity. Signals subjected to such modulation have a wide dynamic range, infrequently achieve peak levels, and frequently operate below peak levels. To provide linear amplification for signals subjected to such modulation, traditional radio frequency (RF) amplifier architectures need to be significantly backed-off. This generally results in poor efficiency.
A number of techniques exist to improve efficiency based on supply voltage. Notable amongst these supply voltage based efficiency enhancement schemes are those of envelope tracking and envelope elimination and restoration.
These two techniques improve efficiency by dynamically adjusting or tracking the supply to the amplifier device in harmony with the envelope of the modulation signal to be amplified. When applied to an amplifier stage, the dynamic adjustment of the supply alters the drain supply or bias of the amplifier. This results in high final stage efficiencies.
At least three problems exist in implementing such dynamic supply adjustment techniques. Firstly the need to modulate the supply may require tracking bandwidths of three to four times the modulation bandwidth of the signal to be amplified, which must be generated efficiently. Secondly, any noise or distortion in the supply to the amplifier will be modulated up to the carrier frequency resulting in unacceptable out-of-band spectral emissions. Finally, the supply typically requires a high output voltage dynamic range and is typically required to deliver a high peak to average power ratio.
As a result a supply voltage modulation source not only needs to be efficient, but must also have a tracking response which is substantially accurate; has a high slew rate and peak power; and produces distortion that is either small or bandwidth constrained and predictable.
When applied to high power broadband systems such as multi-carrier WCDMA (wideband code division multiple access), prior art attempts to address this problem using either Class-S modulators (pulse width modulation) or Class-G modulators (switched supply) exhibit unacceptable performance due to insufficient tracking bandwidth, excessive switching losses, or unacceptable distortion characteristics.
It is an aim of the invention to provide an improved technique which addresses one or more of the above-stated problems. In particular it is an aim of the invention to provide an improved supply architecture, which offers improved or optimised efficiency.